Hello!
So basically, the problem is stated in title - the mouse scroll is not available to be bound on ‘Button’ actions. It’s available only to axis or vector action types represented as mouse scroll y for default vertical and x for horizontal scroll.
This is understandable behavior for using mouse scroll in scenarios like alternative binding to touchscreen swipe or gamepad stick in the same vector/axis-type action. Also, it quite usable for trackpads and kind of smooth scroll mice like Apple’s one or Logitech’s with unlockable smooth wheel.
But for most PC gaming scenarios mouse wheel is still using in it’s stepped rotation behavior where single step defined as single press for some action. For example, in shooter games it’s common to use mouse wheel to swap weapons. And when you rebinding some actions to mouse wheel in other games you can see the direction of wheel on binding icon which is quite important.
Although it is still possible to implement mouse wheel to perform some button-like actions in code there will be some limitations to use it in new Input Systems. For instance, I’m defining icons of bindings by binding path and as far as there is no such path like <Mouse>/scroll/up I can’t show it’s in result. Moreover, I won’t be able to even rebind some button-type actions’ bindings onto mouse wheel at all.
Therefore, we really need to have mouse wheel direction as acceptable path for button-type actions’ bindings. We already have similar behavior of gamepads’ sticks where <Gamepad>/leftStick/up is acceptable path for buttons even though sticks have smooth axis input.